1) Field of the Invention
The present invention relates to an image display apparatus using a current-controlled light emitting element, and more particularly to an active matrix image display apparatus which controls brightness uniformly across its display unit.
2) Description of the Related Art
An organic electro-luminescence (EL) display apparatus using a self-luminous organic EL element (Organic Light Emitting Diode) does not need a backlight, which is required for a liquid crystal display device. This organic EL display apparatus is therefore suitable for a thin display device, and its practical use as a next generation display device has been expected because of no restriction in a view angle.
An image display apparatus using the organic EL element can employ a simple (passive) matrix type and an active matrix type as a drive system. The former has a simple configuration but has a difficulty in realization of a large-size and high-definition display. Therefore, in recent years, the active matrix display device has been extensively developed. The active matrix display device controls a current passing through a light emitting element inside a pixel by an active element, for example, a thin film transistor provided in the pixel.
Shown in FIG. 9 is a pixel circuit in an active matrix organic EL display device according to the conventional technology. The pixel circuit according to the conventional technology includes an organic EL element 105 whose positive side is connected to a positive power supply Vdd, a thin film transistor 104 whose drain electrode is connected to a negative side of the organic EL element 105 and source electrode is connected to the ground, and that functions as a driver element. The pixel circuit also includes a capacitor 103 that is connected between a gate electrode of the thin film transistor 104 and the ground, and a thin film transistor 102 whose drain electrode is connected to the gate electrode of the thin film transistor 104, whose source electrode is connected to a data line 101, and whose gate electrode is connected to a scan line 106, and that functions as a switching element.
The operation of the pixel circuit is explained below. When the potential of the scan line 106 is at a high level, the thin film transistor 102 becomes on-state, and by applying a write potential to the data line 101, the capacitor 103 is charged or discharged, and a predetermined potential is written to the gate electrode of the thin film transistor 104. Next, When the potential of the scan line 106 is at a low level, the thin film transistor 102 is not energized, and the scan line 106 and the thin film transistor 102 are electrically disconnected, but the gate potential of the thin film transistor 104 is stably maintained by the capacitor 103.
The current passing through the thin film transistor 104 and the organic EL element 105 becomes a value corresponding to a gate-source potential Vgs of the thin film transistor 104, and the organic EL element 105 continues light emission with a brightness corresponding to the current value. By performing a write with a potential once in the pixel circuit as shown in FIG. 9, the organic EL element 105 continues light emission with a constant brightness until the next write is performed (e.g., see Japanese Patent Application Laid Open No. H8-234683).
Incidentally, the thin film transistor 104 that functions as the driver element in the image display apparatus includes a channel layer for which polycrystalline silicon or amorphous silicon is generally used. In the image display apparatus in which many pixels are arranged and many driver elements are provided corresponding to the respective pixels, it is preferable to use amorphous silicon in order to suppress variations in characteristics among thin film transistors.
However, when the thin film transistor whose channel layer is formed of amorphous silicon is used as the driver element, there exists a problem such that the conventional image display apparatus as shown in FIG. 9 has a difficulty in keeping high-quality image display over a long period of time. This is because in the thin film transistor using the amorphous silicon, it is known that a threshold voltage varies gradually by passing a current through the channel layer over the long period of time and that the value of the current passing through the channel layer changes according to the variations in the threshold voltage even if it is continuously applied with a constant gate voltage. As explained above, the organic EL element 105 is serially connected to the thin film transistor 104, and therefore, the value of the current passing through the organic EL element 105 changes according to the variations in the value of a current passing through the channel layer. Therefore, even if the same potential is supplied from the data line 101, the brightness of the organic EL element 105 varies according to variations in the threshold voltage, which makes it difficult to display a high-quality image.
Therefore, in an actual image display apparatus in which the thin film transistor using the amorphous silicon is used as the driver element, a voltage compensation circuit is arranged for each pixel in addition to the pixel circuit as shown in FIG. 9. More specifically, in addition to a potential that is supplied from the data line 101, a potential for compensating for a variation of the threshold voltage is supplied from the voltage compensation circuit to the gate electrode of the thin film transistor 104. This configuration allows high-quality image display. However, such a voltage compensation circuit includes a few pieces of thin film transistors per pixel, which causes an area for the voltage compensation circuit to be separately provided on a substrate where the organic EL elements are arranged. Consequently, the organic EL elements 105 cannot be densely arranged thereon, which causes occurrence of a new problem such that it becomes difficult to perform high-definition image display.
Furthermore, it is known that degradation in the channel layer causes not only the threshold voltage of the thin film transistor 104 but also the current passing according to the gate potential to change, so-called a slope of a linear area to vary. Although the brightness is affected by the variation of the slope of the linear area less than the variation of the threshold voltage, the variation of the slope should not be neglected in order to perform high-quality image display.